A polyester film is produced, generally, by casting a molten polyester sheet, delivered from a die onto the surface of a mobile cooling medium that is continuously moving so as to solidify the molten polyester sheet by cooling, heating the solidified sheet and stretching the sheet in its longitudinal direction. The film stretched in the longitudinal direction is a uniaxially stretched film. If the film is further stretched in its transverse direction, after stretching it in the longitudinal direction, a biaxially stretched film can be obtained.
As high-speed film forming techniques in the production of polyester film, a method for increasing the casting speed by casting a molten polyester sheet onto the surface of a mobile cooling medium having a liquid layer, such as a water layer on the surface, and a method for increasing a longitudinal draw ratio by stretching a sheet in its longitudinal direction through a plurality of stretching steps not less than two steps, are individually known, respectively.
The former method is disclosed in, for example, U.K. Patent 1,140,175 and JP-B-SHO-58-35133.
However, there exists a limit to the increase in the film forming speed after the longitudinal stretching of a sheet only by cooling and solidifying a molten polyester sheet on the surface of a mobile cooling medium having a liquid layer on the surface. Moreover, in this casting process, it is important to form a uniform liquid layer on the surface of a mobile cooling medium and interpose the formed liquid layer uniformly between the surface of the mobile cooling medium and the molten polyester sheet. If the liquid layer is not uniformly formed or interposed, it causes the cooled and solidified sheet to have surface defects.
The latter method, i.e. the method for stretching a polyester sheet in its longitudinal direction with a high stretching ratio through a plurality of stretching steps, is disclosed in, for example, JP-B-SHO-52-10909 and JP-B-SHO-52-33666. These publications teach a process for stretching a polyester sheet at a total draw ratio of not less than 4.5 times the original length by multi-stage stretching comprising a high-temperature stretching process and a low-temperature stretching process. Since this multi-stage stretching can achieve a high longitudinal draw ratio, such as a ratio of about two times of 2.5-3.5-fold which is disclosed in JP-B-SHO-38-23489 etc., or a ratio higher than that value such as 5-9 times, high-speed film formation process can be attained, including speeds at greater than 200 m/min.
In this multi-stage stretching, however, it is necessary to increase the stretching temperature in comparison with a usual stretching temperature of 80.degree.-95.degree. C. in order to ensure a high longitudinal draw ratio. Therefore, in this stretching process, problems are liable to occur such as (1) the thickness variation of the film obtained deteriorates because the draw ratio is likely to disperse, (2) the surface of the film obtained becomes rough because the film is likely to adhere to a high-temperature stretching roll, and (3) the abrasion resistance of the surface of the film finally obtained decreases because the degree of crystallinity of the film increases and the surface layer of the film is likely to be abraded. Moreover, because the polyester sheet (or film) is stretched at a high temperature, an oligomer precipitates from the sheet. The precipitated oligomer soils the surface of a stretching roll in a short period of time. If the surface of the stretching roll is soiled, surface defects occur on the obtained film, and the productivity of the film is greatly decreased as the soiled roll must be cleaned or exchanged. Furthermore, in this conventional multi-stage stretching, if the film is stretched at a total draw ratio of not less than 4.5 times the original length at a temperature of 80.degree.-95.degree. C. which is a usual stretching temperature, the film formation is not stable and quality defects are caused such as a rough surface and degraded transparency.